In general, a lithographic printing plate precursor comprises a lipophilic image area which receives an oil-based ink at the printing step and a hydrophilic non-image area which receives dampening water at the printing step. Lithographic printing is a printing method utilizing the repulsion between water and oil-based ink. In some detail, the lipophilic image area on the lithographic printing plate acts as an ink-receptive area while the hydrophilic non-image area acts as a dampening water-receptive area (ink-nonreceptive area). The surface of the lithographic printing plate is made different in ink affinity from area to area. An ink is then attached to the image area alone. The ink is then transferred to printing material such as paper to effect printing.
In order to prepare lithographic printing plates, a lithographic printing plate precursor (PS plate) comprising a lipophilic photosensitive resin layer (image recording layer) provided on a hydrophilic support has heretofore been widely used. In general, the lithographic printing plate precursor is exposed to light through an original such as lithographic film. The image area on the image recording layer is left undissolved while the other unnecessary image recording layer is dissolved away with an alkaline developer or organic solvent with. In this manner, the surface of the hydrophilic support is exposed to form a non-image area and obtain a lithographic printing plate.
The related art process for the production of light-transmitting requires a step of dissolving the unnecessary area away with a developer or the like after exposure. One of the recent technical assignments in the art is to eliminate or simplify a wet process which is additionally conducted as in the development step. In recent years, the disposal of waste liquid discharged from the wet process has become a great concern to the entire industry taking into account the global environment. With such an environmental problem, the demand for elimination of wet process has been growing more and more.
As a simple plate-making method there has been proposed a method called on-the-machine development method using an image recording layer which can be freed of unnecessary areas at an ordinary printing step. In some detail, after exposure, unnecessary areas are removed from the image recording layer on the printing machine to obtain a lithographic printing plate.
Specific examples of the on-the-machine development method include a method involving the use of a lithographic printing plate precursor comprising an image recording layer capable of being dissolved or dispersed in a dampening water, ink solvent or an emulsion of dampening water and ink, a method involving the dynamic removal of an image recording layer by contact with the rollers or blanket cylinder of the printing machine, and a method involving the dynamic removal of an image recording layer by contact with the rollers or blanket cylinder of the printing machine after the reduction of the cohesive force of the image recording layer or the adhesion between the image recording layer and the support by the penetration of dampening water, ink solvent, etc.
However, the related art method involving the use of an image recording layer of image recording process utilizing ultraviolet rays or visible light requires a troublesome procedure such as storage of exposed lithographic printing plate precursor under completely light-screened or constant temperature conditions until mounting on the printing machine because the image recording layer is not fixed even after exposure.
In recent years, on the other hand, a digitization technique involving electronic processing, storage and output of image data using computer has been widely spread. Various new image output methods that cope with this digitization technique have been put to practical use. With this technical trend, a computer-to-plate technique has been noted which comprises scanning a lithographic printing plate precursor with a highly convergent radiation such as laser beam having digitized image data carried thereon to produce a lithographic printing plate directly without lithographic film. Accordingly, one of important technical assignments is to obtain a lithographic printing plate precursor adapted for such a technique.
As mentioned above, it has recently been desired more keenly than ever to simplify the plate-making job, effect the plate-making job in a dry process and eliminate the disposal step from the plate-making job taking into account both global environment and adaptation to digitization.
Since high output lasers such as semiconductor laser emitting infrared rays having a wavelength of from 760 nm to 1,200 nm and YAG laser have been recently available at reduced cost, a method involving the use of these high output lasers as an image recording means has been considered a promising method of producing a lithographic printing plate by scanning exposure, which can be easily incorporated in digitization technique.
The related art plate-making method comprises imagewise exposing a photosensitive lithographic printing plate precursor to light at a low to middle intensity so that the resulting photochemical reaction in the image recording layer causes imagewise change of physical properties by which image recording is conducted. On the contrary, the aforementioned method involving the use of high output laser comprises irradiating the exposure area with a large amount of optical energy in an extremely short period of time so that the optical energy is efficiently converted to heat energy by which a thermal change such as chemical change, phase change and morphological or structural change occurs in the image recording layer, and then using this change to record image. Accordingly, image data is inputted in the form of optical energy such as laser beam, but image recording is effected taking into account reaction by heat energy in addition to optical energy. In general, such a recording method utilizing heat generation by high power density exposure is called “heat mode recording” and the conversion of optical energy to heat energy is called “photothermal conversion”.
A great advantage of the plate-making method involving the use of heat mode recording is that the image recording layer cannot be exposed to light having an ordinary level of intensity such as indoor illumination and the fixation of image recorded by high intensity exposure is not essential. In other words, the lithographic printing plate precursor for use in heat mode recording cannot be exposed to indoor illumination before required exposure and it is not essential that images formed by exposure be fixed. Accordingly, the execution of a plate-making step of imagewise exposing an image recording layer capable of being insolubilized or solubilized by exposure to light from a high output laser on the printing machine to prepare a lithographic printing plate allows a printing system that gives an image insusceptible even to exposure to indoor ambient light. It can be therefore expected that the utilization of heat mode recording makes it possible to obtain a lithographic printing plate precursor suitable for on-the-machine development.
However, most of photosensitive recording materials which can be practically used as image recording layers are sensitive to visible light in the light wavelength range of 760 nm or less and thus cannot record image when irradiated with infrared laser beam. It has thus been desired to provide a material capable of recording image when irradiated with infrared laser beam.
To this end, Japanese Patent No. 2,938,397, for example, proposes a lithographic printing plate precursor comprising an image-forming layer having a particulate hydrophobic thermoplastic polymer dispersed in a hydrophilic binder provided on a hydrophilic support. Japanese Patent No. 2,938,397 discloses that when exposed to infrared laser beam, this lithographic printing plate precursor generates heat that causes the hydrophobic thermoplastic polymer particles to undergo coalescence resulting in the formation of an image. The lithographic printing plate is then mounted on the cylinder of the printing machine on which it can be developed with a dampening water and/or ink.
The aforementioned image forming method involving the coalescence of particles by mere heat fusion gives a good on-the-machine developability but is disadvantageous in that the resulting image recording layer exhibits an extremely poor image strength (adhesion to support) and hence an insufficient press life.
Further, JP-A-2001-277740 and JP-A-2001-277742 disclose a lithographic printing plate precursor comprising microcapsules having a polymerizable compound encapsulated therein incorporated in a hydrophilic support.
Moreover, JP-A-2002-287334 discloses a lithographic printing plate precursor comprising a photosensitive layer (image recording layer) containing an infrared absorbent, a radical polymerization initiator and a polymerizable compound provided on a support.
Thus, the method involving the use of polymerization reaction provides an image area having a higher chemical bond density and hence a relatively higher image strength than that formed by heat fusion of polymer particles but leaves something to be desired all in on-the-machine developability, press and polymerization efficiency (sensitivity) and stainproofness from the practical standpoint of view and thus has not yet been practically used.
In the related art of lithographic printing plate precursor comprising a photosensitive polymerizable composition layer, a technique which comprises providing a undercoating layer containing a polymerizable compound in the interface of the photosensitive polymerizable composition layer with the support to exert an effect of enhancing press life is known in, e.g., Japanese Patent No. 3,475,377 and JP-A-10-260536. However, these techniques require that the undercoating layer be removed by alkaline development. Accordingly, in the case where development is effected with an oil-based ink and an aqueous component on the printing machine rather than with alkali, the undercoating layer remains on the non-image area, causing the occurrence of stain on the area which is not desired to have ink attached thereto. Thus, desired printed matters cannot be obtained.
JP-A-2004-9482 discloses a technique of spreading a silicate-based glass as a undercoating layer in a method of forming an image with an oil-based ink and an aqueous component on the printing machine. The provision of such a silicate-based glass makes it possible to enhance the hydrophilicity of the surface of the support and enhance the image-forming properties but causes drastic drop of the adhesion between the photosensitive layer and the support, giving a tendency for deterioration of press life.
Further, U.S. Pat. No. 6,014,929 discloses a lithographic printing plate comprising an interlayer provided on a mechanically-interlockable support which interlayer can be separatable when dissolved or dispersed in a dampening water or ink. In general, however, a hydrophilic resin which can be separated with a dampening water or aqueous component makes it possible to enhance image-forming properties as mentioned above but causes drastic drop of the adhesion between the photosensitive layer and the support, giving a tendency for deterioration of press life. Further, the resin exemplified in Patent Reference 8 exhibits a high affinity for a hydrophilic support and thus remains during printing, causing undesirable staining on the non-image area or making it difficult to maintain printing unless excessive dampening water is supplied. It was further made obvious that there occur practically serious defects such as unstable ink density and incapability of removing ink from the non-image area when the operation of the printing machine is once suspended to confirm the quality of printed matters and then resumed.